In many applications, particularly precision optical systems such as are used in semiconductor processing, inspection and testing, it is desirable to be able to move an object very precisely with respect to two orthogonal axes while maintaining the object in a reference plane. Step and repeat projection systems provide a good example of the need for a positioner or aligner that must meet such critical and difficult requirements. Such systems are used in the semiconductor industry for repeatedly projecting the image of a photomask onto a wafer at different positions. The array of patterns on a silicon or other semiconductor wafer must be successively laid down in columns and rows with submicron accuracy. Semiconductor wafers of 6" diameters are now being utilized and up to 8" diameters will be used in the future.
Because constantly smaller images are being generated, ever greater accuracy in positioning is being demanded. With 6" wafers, for example, high density circuit patterns are laid down at center-to-center spacings of the order of 0.10". Thus approximately 2500 images must be exposed or processed sequentially with a positioning accuracy of 0.1 micron in order that the complete processing sequence, which typically involves many projection steps, can be carried out with precise alignments and high yields. It is also sometimes required to be able to rotate the image through a small angle (e.g. 3.degree.) in order to align the pattern for various purposes.
These requirements for accuracy and reliability of operation are such that older bearing mounted positioners with compounded XY stages cannot be as precisely level as needed, much less provide rapid but highly precise movement between successive exposure positions. It has been recognized that a platform supported on air bearings above a stable, precisely flat surface might achieve suitable positioning speed and accuracy. However, this remains no more than a desirable goal at the present time, inasmuch as an air bearing mounted XY drive system with limited angular motion is not yet commercially available. Moreover, there are many potential problems to be considered, such as the settling time required before the platform is truly stationary once it has been moved to a new position. With most XY stages, driving thrusts are exerted off axis relative to the center of gravity of the platform. Thus overturning moments are exerted that would induce a rocking action at some resonant frequency in air bearing mounts. Until this rocking action becomes damped to an adequately low level the next projection step cannot be undertaken.
Linear motors are well known as devices for providing precisely controllable longitudinal movement. Because the elements of the linear motor lie along a plane, and the motor can move the elements at high speed, its promise for use in precision positioning systems is recognized. Precision sensors employing laser interferometers can be used to provide signals for servoing position to submicron accuracy. However, no suitable configuration of two axis or X-Y linear motor drive is available or known. The added requirements of a semiconductor processing station, including the ability to maintain the image in an almost perfectly flat plane, rotate the image through a small angle, and position rapidly with minimal settling time, also remain to be met.
A two-dimensional linear stepping motor system was initially proposed by Sawyer in U.S. Pat. No. Re.27,436, and has been used since principally for controlling the mechanism in a flatbed plotter. In this system two orthogonally disposed linear stepping motors are defined within a movable platform above a planar base. The platform can be actuated in X or Y separately or in a vectorially combined direction, but only in stepping fashion. Even using known microstep techniques, however, this mechanism cannot meet the precision and velocity requirements of the semiconductor industry without much refinement and excessive cost. It also cannot rotate relative to the XY plane, and thus would require an added mechanism for this purpose.
A system for positioning an element in two dimensions in semiconductor manufacturing apparatus is disclosed in recently issued U.S. Pat. No. 4,535,278 to Asakawa. In this proposal an array of permanent magnets is arranged in a spaced apart rectangular pattern, with magnets of like polarities disposed along diagonals. Driving coils in the form of flat, square sided, loops are mounted on the underside of a movable member. Four such coils are shown, with outer dimensions, widths and spacings being precisely related to the particular magnetic fields of the array so as to generate predictable instantaneous force vectors, depending on incremental position and instantaneous driving current, for each coil. By manipulating individual driving currents constantly as position changes, net drive force vectors are said to be generated that induce the needed motion in the driven device. This system is said to be capable of controlling angular position as well as X and Y positions. It is, however, extremely complex because it is based on multiple non-linear interactions which vary with position. The four sided coils may interact at any instant with from none to four permanent magnets to give a predictable but highly variable resultant force vector. Each of the four sides of a coil sees a different instantaneous force vector that varies in both angle and amplitude and must be resolved by signal processing into individual driving signals that somehow yield the desired net X and Y forces. Thus the driving current for each coil must constantly be computed and changed, requiring input signals to be converted depending on position in accordance with complex "distribution factors" in "multiplier type digital-analog converters". For high precision the system requires virtually infinite resolution, and it appears likely that some positions may exist at which pure X and Y forces cannot be resolved. Furthermore, the forces exerted are not symmetrically applied relative to the center of the device and settling times are highly uncertain.